FIG. 1 illustrates a conventional LED die 10. The LED die 10 includes semiconductor layers and, optionally, a transparent substrate, represented by the LED layers 12. The bottom cathode electrode 14 is electrically connected to the n-type layers in the LED die 10, and the anode electrode 16 is connected to the p-type layers in the LED die 10. Although the LED die 10 is shown as a flip-chip, it may instead have one wire-bonded top electrode or two wire-bonded top electrodes.
The LED die 10 can initially be mounted on a thermally conductive submount 18, such as having an AlN body. Using a submount greatly eases handling of the LED die 10 and mounting on a circuit board. The submount 18 has top metal pads 20 and 22 that are bonded to the LED die electrodes 14 and 16 via ultrasonic welding or other technique. The submount's metal pads 20 and 22 are electrically connected to bottom electrodes 24 (only one is shown in the cross-section) by means of metal traces on the top surface of the submount 18 and vertical vias 26 extending through the submount body.
The submount 18 acts like an interface between the LED die 10 and the printed circuit board 30 for conducting electrical current and the heat generated by the LED die 10. The heat generated by the LED die 10 is primarily conducted to the submount body by the LED die electrodes 14 and 16, and the submount 18 conducts the heat to the circuit board 30 via an electrically insulated thermal pad 32, usually formed of copper. Electrodes 34 (only one is shown in the cross-section) on the circuit board 30 are soldered to the bottom electrodes 24 of the submount 18, and the thermal pad 35 on the circuit board 30 is soldered to the thermal pad 32 on the submount 18. The electrodes 34 are connected by traces 36 to a power source. The circuit board 30 is often highly thermally conductive, such as formed of an aluminum or copper core with a thin dielectric layer 38 to electrically insulate the electrodes 34 and traces 36 from the metal core.
The thermal pad 35 on the circuit board 30 may be much larger than the footprint of the submount 18 to spread the heat laterally.
FIG. 2 illustrates the electrodes 24A and 24B and thermal pad 32 on the bottom surface of the submount 18, which correspond to the electrode and thermal pad pattern on the circuit board 30.
Problems with such a design are that the heat spreading within the Cu layer of the circuit board 30 is partially choked by the location of the electrodes 34, since the thermal flow is interrupted by the gap between thermal and electrical pads. Another problem is that the molten solder 40 (such as from a solder bath) that wets over the thermal pad 35 is not flat due to surface tension, and the height of the molten solder 42 on the smaller electrodes 34 is different from the height of the solder 40 over the thermal pad 35. This results in the submount 18 and LED die 10 being slightly tilted when mounted over the circuit board 30 after the solder is solidified by cooling. This effect comes especially into play for larger submounts, such as larger than 2.5×2.5 mm.
Further, in the design of FIGS. 1 and 2, solder cracks and delamination, resulting from a mismatch in the coefficient of thermal expansion (CTE) between the submount 18 and the circuit board 30, are more likely to occur because the stresses increase with distance from the center of the submount 18, and the small electrodes 24 are furthest from the center of the submount 18. If the electrodes are not adequately electrically connected together by the solder, arcing can occur for a cracked open contact, high heat can be generated at the interface, and there will be an increased voltage drop.
The same issues occur if the bottom surface of the LED die 10 had an electrode and thermal pad configuration similar to that shown in FIG. 2, and the LED die 10 were directly bonded to corresponding pads on a circuit board.
FIG. 3 illustrates another conventional configuration of electrodes 44 and 46 and a thermal pad 48 on the bottom surface of a submount for an LED die. This 3-stripe design has drawbacks for tilting, choking of heat spreading by the electrodes 44/46, and solder cracking/delamination that are similar to those described above for the configuration of FIG. 2, since the electrodes 44 and 46 are away from the center and partially surround the thermal pad 48.
What is needed is a metal electrode and thermal pad configuration on the bottom surface of an LED die or submount that does not suffer from the drawbacks described above.